That's an interesting question. I think it's probably because AI is essentially an interpolation exercise. They are bland, because AI averages things it sees as correlated with the prompt. Truely creative things are usually a little on the edge of normal and make you go .. huh... that's kinda cool cause I haven't seen it before. AI is exceptional at making things you have seen before. Also, All AI stuff is always a little malformed. AI doesn't 'understand' what a circle is. I can make circles, but the true perfection of a circle is lost on vector matrix.
It's why I embrace AI. Its great for some task, like making some sprites for my hobby project. It will never beat a true professional who understands human feelings, desires, and a real world context.